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kaidenlem

FUEL PUMP WILL NOT BUILD PRESSURE AND COMANCHE WILL NOT START

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Having some trouble with my Comanche. The fuel pump was just replaced yesterday, and now it will not shut off when the key is on, and the vehicle will not start. It’s an 87 with the 4.0. Any suggestions? Fuel regulator?

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Pull the Fuel Pump Relay out. Does that shut off the fuel pump?

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1 minute ago, Ωhm said:

Pull the Fuel Pump Relay out. Does that shut off the fuel pump?

What color wire is it and where is the relay located? 

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Relays are located on the right side of the engine compartment. Can't find a photo. Need some help here.

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Check pressure at the fuel rail. If the pump doesn't build pressure, it won't shut off because the regulator won't tell it to shut off.

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Is there actual electrical feedback to the regulator? My understanding is it's just a spring and diaphragm setup with vacuum signal from the intake, and that the pump just runs for a couple seconds regardless for priming on key-on.

 

A pump that doesn't shut off would indicate an electrical problem. Usually problems immediately after a fuel pump is changed out are either a new filter that was installed backwards (they typically have check valves) or else the small rubber hose from the pump to the sending unit (inside the tank) failed. 

But an electrical issue causing it not to turn off if it worked fine before hand... (or did it?) If the fuel gauge wires are a switched ground, is there a chance the harness for the sending unit was damaged during the swap and now the gauge and power wires are shorting against each other?

 

Why was the fuel pump changed in the first place? Was anything else done at the same time that could interfere with a diagnosis?

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1 hour ago, gogmorgo said:

Is there actual electrical feedback to the regulator? My understanding is it's just a spring and diaphragm setup with vacuum signal from the intake, and that the pump just runs for a couple seconds regardless for priming on key-on.

 

A pump that doesn't shut off would indicate an electrical problem. Usually problems immediately after a fuel pump is changed out are either a new filter that was installed backwards (they typically have check valves) or else the small rubber hose from the pump to the sending unit (inside the tank) failed. 

But an electrical issue causing it not to turn off if it worked fine before hand... (or did it?) If the fuel gauge wires are a switched ground, is there a chance the harness for the sending unit was damaged during the swap and now the gauge and power wires are shorting against each other?

 

Why was the fuel pump changed in the first place? Was anything else done at the same time that could interfere with a diagnosis?

 

To me, a pump that doesn't shut off combined with no fuel suggests a pump that's not pumping. The regulator on the Renix models, IIRC, allows over-pressure to be routed back to the fuel tank. The pump is supposed to shut off when the fuel system reaches a pre-set pressure. That's why the pump shuts off after a few seconds of priming when you go to start the engine. It's not on a timer -- it reacts to the pressure in the system.

 

So ... the engine won't start, and the fuel pump won't stop. Possible correlation, which is why I suggested checking for pressure in the fuel rail.

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30 minutes ago, Eagle said:

The pump is supposed to shut off when the fuel system reaches a pre-set pressure. That's why the pump shuts off after a few seconds of priming when you go to start the engine. It's not on a timer -- it reacts to the pressure in the system.

 

The ECU controls the Fuel Pump using the Fuel Pump Relay. At KEY ON the ECU will ground the fuel pump relay (coil) which will turn the fuel pump ON. If the ECU fails to see the CRANK signal within 2-3 seconds, the ECU will turn the fuel pump relay OFF thereby shutting the fuel pump OFF.

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Found out the fuel pump relay was bad and replaced it, now there is a completely different no spark issue...

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Take a deep breath and then a few hours to complete Tips 1 through 5 at www.cruiser54.com. 

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Got it mostly figured out. Unplugged everything one by one and sprayed with electronic cleaner and it started up fine. Every once in a while, it will run on 5 cylinders, but then when restarted it will fire on all. Also, with the new fuel pump in, it almost seems as though the factory reversed the float wires, because the tank is about 3/4ths full, it shows on 1/4, and then when I filled the tank it went to E. How can this be fixed?

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You've got the wrong sending unit in there. HO and Renix applications are backwards compared to each other. 

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